Implicitly define nonlocal names.

If a capitalized name cannot be resolved, a new top-level
variable with its name is implicitly declared. If a real
definition is not found later, a compile time error is raised.

Mutual recursion at the top level works now!

Fix #101. Fix #106.

test/nonlocal/use_in_method_before_definition.wren

@@ -0,0 +1,14 @@
+class Foo {
+  method {
+    IO.print(Global)
+  }
+
+  static classMethod {
+    IO.print(Global)
+  }
+}
+
+var Global = "global"
+
+(new Foo).method // expect: global
+Foo.classMethod // expect: global